Keep it simple

Albert Einstein said ‘genius is making complex ideas simple…not making simple ideas complex’.

Now we’re not claiming genius within the renaissance design office, however, searching for simple solutions to complicated problems is at the heart of our engineering approach and no more so than for the Viadux tower, where we are bringing the equivalent of a 46 storey tower out of a network of grade 2 listed Victorian arches.

Some of the most impressive pictures captured on the project so far have been associated with the deep (2500-3500mm) piled raft. Understanding the structural behaviour of this element is complicated, the team have developed linear and non-linear finite element models, considered soil-structure interaction, adopted 3D strut-and-tie systems and looked at the strength of concrete when its tri-axially confined. However, it’s the simple solutions that have resulted from this work that have made us smile the most.

A key challenge when casting concrete so deep, is that it generates a lot of heat. It’s not the heat itself that’s the problem though, it’s the difference in temperature as you move from the middle to the edges. That is, if the edges cool too fast (a temperature differential exceeding 20-25oC say)…the concrete can crack…and it cracks where you can’t see it. That’s a problem.

So what hi-tech material can you cover the pile cap with to prevent issues? Well in this case it was….sand! Insulating the top of the cap with 30-50mm of sand ensured we could maintain acceptable temperature differentials through the structural element. The team at Mayo Civils also used cast in sensors, so that everyone could monitor the concrete temperature (online and remotely) for the days and weeks after the pour. Of course, the sand thickness could easily be varied if the temperature gradients were heading in the wrong direction. No special training or BBA certification required. Just sand. Simple.

The other simple technology used to control the temperature within the deep pile cap was blast furnace slag (GGBS). A waste product that results from quenching molten iron slag. Stuff you would just want to throw away. This idea is so old though…even the Romans were doing it. You put a pozzolanic material, GGBS or PFA, in with your cement and it reacts with the free lime and makes other cementitious materials, gaining you strength as a secondary reaction. Bottom line…. you need less cement. Less cement means less heat, less carbon…less stress (thermal and design induced).

Mayo’s worked with their knowledgeable supply chain to develop and trial a number of very high content (50-75%) sustainable cements. A number of trials, undertaken under lab and site conditions, allowed us to pin down a 75% GGBS mix that achieved the required C40/50 strength.

It should also be noted that for once working within the undercroft on this project worked in our favour. The thermal mass of the brickwork was found to better regulate the air temperature and to shield the top of the pour from the wind. Every little helps.

Before we got to pouring the pile cap though, all 2500m3 of it, there were a lot more ‘simple’ ideas that made it possible. The existing arches could be used to enable cost effective temporary works to the excavation works. Adopting a staggered/dice arrangement in the pile layout meant rebar could be bundled, moved and laid more quickly. Simplified, repeatable RC detailing, with very small variation in the size or shape of the rebar also contributed to the speed the rebar could be fixed. Simplicity in design leading to efficiency in construction.

We started with a quote from Albert Einstein and we’ll finish with a quote from Renaissance director Helen Gribbon, ‘KISS…Keep it simple s…..’. Please note though, just because something’s simple…doesn’t mean it’s easy. However, it does mean you can bring fantastic structures to life.

written by Gary Robinson